Arc chamber for an ion implantation system

ABSTRACT

The present invention relates to the fabrication of materials and structures having selected mechanical, thermal and electrical properties. More particularly, the invention relates to the use of these materials and structures in ion implantation systems. Structures comprising boron material provide components for use in implanters including arc chambers with which a beam of ions is generated for implantation into a target such as a semiconductor wafer.

RELATED APPLICATIONS

[0001] This application is a divisional application of U.S. Ser. No.09/290,471 filed on Apr. 12, 1999 which is a continuation application ofU.S. Ser. No. 09/002,748 (now U.S. Pat. No. 5,914,494) filed on Jan. 5,1998 which is a continuation application of International ApplicationNo. PCT/US97/17938 filed on Oct. 3, 1997 which is a continuation-in-partapplication of U.S. Ser. No. 08/725,980 filed Oct. 4, 1996 which is acontinuation-in-part of U.S. Ser. No. 08/622,849 filed on Mar. 27, 1996,the teachings of the above applications being incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to methods for fabricatingmaterials and structures having mechanical, thermal and electricalproperties suitable for use in a wide variety of applications.

[0003] Materials fabricated from powder have in the past been fabricatedusing a sintering process but do not have a sufficient density toprovide a product having sufficient mechanical strength or thermalstability.

[0004] For example, boron has a wide variety of uses, but it is adifficult material to form in a desired geometry and is also difficultto machine. Arsenic, phosphorus, antimony and boron are all used asdopants in the fabrication of semiconductor devices. These materials areselectively ionized and implanted using an ion implantation system.These systems have an ion source that is used to generate a beam ofionized particles which are directed onto a target such as asemiconductor wafer. These systems are complex and expensive tofabricate, operate and maintain. A particular problem in the use ofthese ion implanters is the level of impurities generated during usewhich increases maintenance, increases defect density in the materialsproduced and reduces production yield in the manufacture of devices.

[0005] The housing for the ion source in an implanter is often referredto as an arc chamber. Arc chambers have usually been made of graphite,molybdenum or tungsten. These materials contribute to the contaminationof the beam, and consequently, they contaminate the final product.

[0006] In one type of arc chamber electrons are emitted by a cathode,usually by thermionic emission, and accelerated to an anode. Some ofthese electrons have collisions with gas atoms or molecules and ionizethem. Secondary electrons from these collisions can be acceleratedtoward the anode to energies depending on the potential distribution andthe starting point of the electron. Ions can be extracted through theanode, perpendicular to it, or through the cathode area depending uponthe type of source.

[0007] To increase the ionization efficiency of the electrons inelectron bombardment ion sources, several modifications have beenintroduced in existing systems. An additional small magnetic fieldconfines electrons inside the anode and lets them spiral along themagnetic field lines, multiplying on their way to the anode andincreasing the ionization efficiency of the ion source. By using acylindrical anode and a reflector electrode, the electron path isfurther enlarged. Many mass separator ion sources are this type, such asthe Nier, Bemas, Nielsen, Freeman, Cusp and other sources.

[0008] The Bemas ion source, for example, has a rectangular orcylindrical arc chamber positioned in an external magnetic field. Thesource can contain a single-turn helical filament (cathode) at one sideof the arc chamber and a reflector at the other end. Electrons from thecathode are confined inside the anode cylinder by the magnetic field andcan oscillate between the filament and the reflector resulting in a highionization efficiency. Ions are extracted perpendicular to the anodeaxis through a slit of about 2 mm width and about 40 mm length. However,the dimensions can vary, depending on the specific design.

[0009] A continuing need exists for improvements in the field ofmaterials fabrication to provide structures having desired mechanical,thermal and electrical properties. In particular, there is a need forimprovements in ion implantation systems used for the fabrication ofsemiconductor devices.

SUMMARY OF THE INVENTION

[0010] The present invention relates to devices and methods offabricating components for use in ion implantation systems. Moreparticularly, the invention relates to the fabrication of boron arcchambers and other boron components for ion implantation systems. Withthe use of boron components in ion implantation systems a number ofadvantages are realized, including a reduction in contaminants due tothe use of boron instead of other materials such as graphite, molybdenumor tungsten; the enhancement of beam current that can be accommodateddue to the lower level of contaminants; the lighter weight of thesecomponents and the ability to retrofit them onto existing systems aswell as their use in new systems, and the ability to use thesecomponents with the electrical system (e.g. as electrodes) and as asource of boron particles for ionization.

[0011] The refractory metals are problematic for the ion source becausethey are heavy, difficult to fabricate, and highly reactive with borontrifluoride, a gas used in many systems to provide a source of boron forionization.

[0012] Tungsten, for example, is the current preferred material for theBemas ion source but is far from ideal. It is one of the heaviest of allengineering materials having a density of 19.3 gm/cc, is difficult andexpensive to machine, and reacts with boron trifluoride to form anothergas, tungsten hexafluoride. The chemical reaction between fluorine andtungsten not only erodes the interior of the arc chamber but also actsas a material transport mechanism for depositing tungsten metal at otherregions of the chamber. This effect shortens the chamber lifetime andconsiderably alters its interior geometry. Additionally, tungstenhexafluoride formation acts to pump unwanted tungsten ions into theboron beam current, some of which invariably ends up in the target,which is typically a single crystal silicon wafer orsilicon-on-insulator (SOI) structure used for the manufacture ofintegrated circuits.

[0013] Boron is very light, having a density of 2.46 gm/cc (about 13%the weight of tungsten) and therefore, is less demanding on mountingfixtures and is easier to handle. It is also very hard and strong, evenat the elevated operating temperatures of an arc chamber. It is moredurable than graphite and tungsten, which is prone to creep (permanentdisplacement under an applied load). A boron arc chamber enhances thesource beam current by reaction with free fluorine ions in applicationsinvolving the use of boron trifluoride as a source for boron ions.

[0014] Solid boron has not been utilized in semiconductor processingsystems because it is not a conventional engineering material. There arecurrently no known manufacturers of dense boron products, mainly becausespecialized materials techniques are required to form this type ofboron.

[0015] Structures made from boron for use in the fabrication ofimplanter components can be made using several distinct processes. Apreferred embodiment of a method for making such boron structuresincludes providing a mold or die having the desired shape for the partto be fabricated, positioning a boron material such as an amorphousboron powder into the mold, treating the boron powder under selectedconditions of temperature and pressure to crystallize the powder into amore crystalline state to form a solid unitary boron structure, removingthe structure from the mold and machining the structure as necessary. Inmany applications it is desirable to produce a structure having apolycrystalline lattice with an average crystal size in the range of 1to 10 microns. In some applications it is desirable to form a structurehaving a crystal size in excess of ten microns, including single crystalmaterial. In some applications with lower tolerance requirements theremay remain a large population of crystals with diameters of less than 1micron, typically in the range of 0.5-1 micron.

[0016] It is also preferred that the density of the material produced beat least 50% of its maximum (theoretical) density, and preferably in therange of 80-100% in order to increase the mechanical strength andresistance to erosion. A preferred embodiment employs boron having ahigh purity level having an atomic percentage of elemental boron of atleast 95%, and preferably of at least 99.99% or greater.

[0017] The fabrication process can be pressure sintering methods such asuniaxial hot pressing and hot isostatic pressing, or a casting method, asingle crystal growth method, by deposition from the vapor or liquidphase, or by spray forming. The specific technique employed for a givenworkpart will depend upon the requirements for purity, density, andgeometry as well as the mechanical, electrical, optical and/or chemicalcharacteristics desired.

[0018] For certain ion sources, a preferred embodiment of the inventionutilizes boron material as a source of boron to be ionized. Atsufficiently low pressure and high temperature boron sublimes at a ratesufficient to produce a flow of gaseous boron suitable to generate anion beam for implantation under electron bombardment.

[0019] In another preferred embodiment, boron is used as a filament togenerate electrons by thermionic emission which are then accelerated byan electric field to bombard the boron within the arc chamber togenerate the ionized beam. A magnetic field is used to confine ionswithin the chamber until they are extracted through the exit aperture ofthe chamber to form the ion beam. When highly pure boron is heated to asufficient temperature, it becomes highly conductive. Many arc chambers,for example, operate at temperatures at which boron is conductive andcan thus employ boron as electrically conductive components.Alternatively, doped boron structures can be used which are conductiveat lower temperatures, including room temperature. Boron is also highlytransmissive in the infrared region (e.g. 1-8 microns wavelength) of theelectromagnetic spectrum and can be used as an optical window or lens. Aboron window or lens can thus be used with an infrared sensitive camerasuch as a charge coupled device to monitor thermal processes or otherinfrared imaging applications such as infrared radar.

[0020] Other components within the implanter that are exposed to the ionbeam, or which are likely to contaminate the beam, can optionally alsobe made of boron. These can include, without limitation, the extractionelectrode or grid, components of the beam analyzer such as the beam traptarget, beam deflectors, and components of the implantation chamberincluding trays for holding wafers, electrostatic clamps, and robot armsto control movement of objects within the implantation chamber.

[0021] The processes described herein can also be used in themanufacture of dense boron coatings, sputtering targets, for thepreparation of boron coatings for diffusion into other substratesincluding semiconductors, and as diffusion furnace components.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1A is a schematic diagram of an ion implanter embodying theinvention.

[0023]FIG. 1B illustrates an implantation system providing ion beamtrap.

[0024]FIGS. 2A and 2B are cross-sectional top and side views,respectively, of an arc chamber in accordance with the invention.

[0025]FIGS. 2C, 2D, 2E are detailed front, bottom and side views of aboron filament in accordance with the invention.

[0026]FIG. 3A is a cross-sectional view of a multicusp ion source inaccordance with the invention.

[0027]FIGS. 3B and 3C are top and cross-sectional views of an anode ringfor a multicusp ion source embodying the inventions.

[0028]FIG. 4 is a schematic view of a Freeman source in accordance withthe invention.

[0029]FIG. 5A is a process flow diagram illustrating a hot pressingtechnique for fabricating a boron structure in accordance with theinvention.

[0030]FIG. 5B graphically illustrates a pre-compacting process of boronpowder.

[0031]FIG. 5C graphically illustrates a preferred apparatus forconducting hot press under vacuum.

[0032]FIG. 5D is a graphical representation of a process for making aboron structure in accordance with the invention.

[0033]FIG. 6 is a process flow diagram illustrating steps in anisostatic pressing method for the fabrication of components inaccordance with the invention.

[0034]FIG. 7 is a process flow sequence illustrating the steps in asintering method for fabricating components in accordance with theinvention.

[0035]FIG. 8 is a process flow sequence illustrating the steps in acasting method for fabricating components in accordance with theinvention.

[0036]FIG. 9A is a process flow sequence illustrating a method offabricating single crystal boron components for an ion implanter bypulling from a melt.

[0037]FIGS. 9B and 9C graphically illustrate the single crystal growthmethod.

[0038]FIG. 10 illustrates a method of using an ion implanter inaccordance with a preferred embodiment of the invention.

[0039]FIG. 11 is a schematic illustration of a method of fabricatingboron components of a processing chamber by deposition from the vapor orgas phase.

[0040]FIGS. 12A and 12B are top and cross sectional views, respectively,of an ion source housing fabricated in accordance with a preferredembodiment of the invention.

[0041]FIGS. 13A and 13B illustrate top and cross-sectional views,respectively, of a portion of an ion source housing fabricated inaccordance with a preferred embodiment of the invention.

[0042]FIG. 14 is a cross-sectional view of a boron deposited electrodefabricated in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0043] A preferred embodiment of the invention is illustrated generallyin the ion implantation 10 system of FIG. 1A. The system 10 typicallyincludes an ion source 16, a power supply 14, an extracting electrode orgrid 18, a magnetic beam analyzer 20 and a controller 22 which arepositioned within an inner housing 12. Ions are generated at the source16, extracted from the source 16 with electrode 18 and conveyed alongthe ion beam path 15 within the beam housing 24 into the implantationchamber 34. The beam analyzer 20 selectively separates components of thebeam exiting from the source 16 so that only ions of the desired massare directed towards the ion implant chamber 34.

[0044] The source 16 includes an arc chamber housing 25 constructed froma crystalline boron material. There are several types of sources havingdifferent configurations of the components, however, these componentscan include an anode, a cathode, a third electrode or filament, variousports to introduce materials into the arc chamber 25 to be ionized, andan exit aperture through which ions are extracted from the chamber 25.The boron material used in the chamber will vary depending upon theapplication, but the material will usually have a uniform density ofeither a polycrystalline material or a single crystal material.

[0045] Other components of the implanter can include a beam controller26 that can be used to selectively deflect the beam so that the beam canbe scanned across a target 28. The implantation chamber 34 includes asupport 30 for one or more targets or wafers mounted within the chamber34 and a drive mechanism to control movement of the support 30 relativeto the implantation chamber housing 34. The system can be used, forexample in the fabrication of many size wafers from 50 mm up to 300 mmor higher. A robot arm can be used to move targets within the chamberand/or insert or remove targets from the chamber. As described ingreater detail, many components of processing systems can be made usingboron material as well as the arc chamber in order to lower impuritylevels and improve system durability and performance.

[0046]FIG. 1B illustrates another preferred embodiment of animplantation system providing beam trap. Similar to the system in FIG.1A, the implantation system 400 includes a ion beam source 402 whichgenerates ion beam 407 extracted with the help of extracting electrodes414 and passes the beam through an enclosed path 404. The beam isfiltered by a magnetic beam analyzer which guides a portion 408 of thesource beam having appropriate mass. The portion 406 having heavier massis unable to make the turn into the target guide 426 and is directed toa trap 410. The trap includes a beam target or collector 428 and a valve412 to allow removal of trap debris. The magnetic analyzer 418 iscontrolled by a mass controller 416. The desired ion beam 408 is guidedto strike a target 422 which can be mounted on a rotatable disk 420driven by a motor 424.

[0047] A preferred embodiment of an arc chamber of an ion source is ofBernas type and an example is illustrated in FIGS. 2A and 2B,respectively. The housing 50 comprises a boron material made inaccordance with the methods defined in greater detail below. The boronmaterial is preferably a polycrystalline material with the average grainsize being in the range of 1 to 10 microns. The density of the materialis at least 50% of the maximum theoretical density (TD), and preferablygreater than 60% TD for machining purposes. The boron housing uses boronhaving an atomic percentage of at least 95% elemental boron andpreferably at least 99.99%.

[0048] Other preferred embodiments of the housing material can includesingle crystal boron, or alternatively, certain components can be madewith boron compounds such as silicon hexaboride, tungsten boride, boronnitride or titanium diboride which are compatible for use in combinationwith the solid boron structures described herein. The inner surfaces ofthe chamber can also be coated with pure dense boron or these compoundmaterials, with the specific type of material chosen being dependentupon the type of ion source, the desired insulating characteristics andthe gases used within the chamber.

[0049] The components within the chamber can either be the standardmaterials, or more preferably, these can also be formed from the sameboron material as the housing 50. These can include the shields 52, thecathode which serves as a source of electrons, an anode 67 and therepeller 54.

[0050] The cathode filament 58 serves as a source of electrons thatbombards a boron material entering through the port 56 in the bottomwall 55 of the chamber 50 to form boron ions. FIGS. 2C, 2D and 2Eillustrate front, bottom and side views of a boron filament 58. thefilament 58 can include three elements, two posts 73, 75 and crossmember 71. Member 71 fits within a groove 79 in one end of both posts,at the opposite end of both posts 73, 75 are connectors 77 that extendthrough the wall of the chamber and connect to pin connectors to attachthe filament 58 to a power source.

[0051] A portion of the chamber serves as the ion source anode 67. Thecathode 58 can be made of Ta or W. The gas flow is small so the sourcewill operate in a vacuum environment within the arc chamber of about5×10⁻⁴ torr. Alternatively, as described in greater detail below, thefilament, the ion source and/or the cathode can also be boron material.

[0052] The top wall 51 of the chamber 50 has an aperture 62 orrectangular slit through which ions within a containment region 65within the chamber are drawn.

[0053] The filament 58 generates electrons which oscillate between thefilament and the reflector. The electrons impact particles containingboron which have been directed into the electron path to produce boronions. The ions are extracted perpendicular to the anode axis through theaperture 62. Additional ports 69 can be used to introduce gases into thechamber. The boron source 60 can either be a standard gas or a boronmaterial that is heated above its sublimation temperature to produce asufficient flow of boron particles.

[0054] The top 51, bottom 55, sides 53,57, plates 52, and end plates canbe made separately as described hereinafter and bonded at the edges, oralternatively, they can be mounted using an external fixture, or a largenumber of plates and rings can be made separately and pressed together.Alternatively, chamber components can be formed to near net shapewithout the need for extensive further machining.

[0055] In the multicusp ion source, the primary ionizing electrons arenormally emitted from tungsten-filament cathodes. The source chamberwalls or a ring form the anode for the discharge. The surface magneticfield generated by rows of permanent magnets, typically ofsamarium-cobalt or neodymium-iron, can confine the primary ionizingelectrons very efficiently. As a result, the arc and gas efficiencies ofthese sources are high.

[0056] The multicusp ion source is relatively simple to operate. Thereare four main components in the source: the filaments (cathode), thechamber, an anode ring, and the first, or plasma, electrode. A schematicdiagram of the multicusp source is shown in FIG. 3A. The source chambercan be rectangular, square or circular in cross section and is formedwith a boron material. The permanent magnets can be arranged in rowsparallel to the beam axis. Alternatively, they can be arranged in theform of rings perpendicular to the beam axis. The back plate alsocontains rows of the same permanent magnets. Grooves are generallymilled on the external wall so that the magnets are mounted withinapproximately 3 mm from the vacuum. These magnets generate multicuspmagnetic fields for primary electron as well as for plasma confinement.

[0057] The open end of the source chamber is closed by a set ofextraction electrodes which can be made with a conductive boronmaterial. The source can be operated with the first electrodeelectrically floating or connected to the negative terminal of thecathode. The background gas is introduced into the source chamberthrough a needle or a pulsed valve. The plasma is produced by primaryionizing electrons emitted from one or more boron or tungsten filaments,which are normally biased negatively with respect to the chamber wall orthe anode ring. These filaments are located in the field-free region ofthe ion source chamber and they are mounted on molybdenum holders. Theplasma density in the source, and, therefore, the extracted beamcurrent, depends on the magnet geometries, the discharge voltage andcurrent, the biasing voltage on the first extraction electrode, and thelength of the source chamber. The source chamber is normally pumped downto about 10⁻⁶ to 10⁻⁷ torr before gases are introduced for beamgeneration.

[0058] The multicusp source for ion implantation and for surfacemodification purposes, beams of B⁺, P^(×), As⁺, and N⁺ ions have beenextracted from multicusp ion sources. By operating the source at higherdischarge voltages, ions with multiply charged states have beengenerated. FIG. 3A and 3B illustrate top and side cross-sectional views,respectively, of a boron anode ring for a multicusp ion source. As theanode ring operates at a temperature of about 600° C., the anode isconductive.

[0059] Freeman developed another type of source by putting the cathodeof a Nier type mass separator ion source inside the hollow anode, as ina magnetron, but left the magnetic field low. The performance of theFreeman ion source has made it successful for ion implantation andindustrial application, especially for semiconductor implantation.

[0060] The Freeman type ion source as shown in FIG. 4 has a similardesign to a magnetron, but it uses just a low external magnetic field ofabout 100G. The arc current is 1 to 3A and the arc voltage is between 40and 70 V. A rigid cathode rod 80, usually 2 mm in diameter and made oftantalum or tungsten, is heated with about 130 A and a few volts to theright temperature. The cathode 80, as well as the chamber 82 can be madeof boron. The axial position of the cathode in the Freeman ion sourceshows several advantages including:

[0061] 1. The inherent magnetic field of the cathode forces the primaryelectrons to move around the cathode, concentrating the electron densityin this area.

[0062] 2. The high electron density next to the extraction slit producesa high ion density in this area, and, thus, a high ion beam current.

[0063] 3. The straight-filament rod fixes the plasma parallel to themagnetic field lines and the extraction aperture, which is the reasonfor the excellent beam quality of Freeman ion sources.

[0064] 4. The low magnetic field does not force instabilities like thehigh field in a magnetron.

[0065] The lifetime of the source is dependent upon the type of gas usedor corrosion rate of the boron or other components in the source.Changing the polarity of the filament after some time of operationimproves cathode lifetime. Heating by AC has the same effect butincreases plasma instabilities and the energy spread of the extractedions.

[0066] The Freeman ion source is especially designed to deliver ionbeams from nongaseous materials. There are many versions with ovens forvarious temperatures and for the application of chemical compounds andin situ chemical synthesis of the required material. With chemicalcompounds, corrosion problems occur not only at the cathode, but also atthe anode and with the oven.

[0067] Ion currents of several milliamps can be produced for mostelements and more than 20 Ma for a few elements like arsenic andphosphorus under favorable circumstances and using large extractionareas. The extraction slit is usually about 2 mm wide and about 40 mmlong. Larger slits are possible, such as 90 mm, but there are somedisadvantages because the current density is not uniform along the longslit due to the bigger voltage drop along the cathode. The anode and themagnetic field can be adjusted, however, to overcome this problem.

[0068] The ion current density is controlled by the arc current, whichis controlled by the filament, the gas pressure, and the magnetic field.The ion source is mounted inside the vacuum chamber. The feed-throughscan be mounted on a base flange perpendicular to the source axis. Theextraction slit is usually 40×2 mm but designs up to 100×5 mm have beenrealized. This type of source can use a W, Ta or boron filament, 1.5-2.5mm φ; alumina or boron nitride insulators; and operates in a vacuum atless than 7.5×10⁻⁶ torr.

[0069]FIG. 5A describes the process flow sequence of a preferred hotpressing method. The process begins at 200 by preparing boron of 99.9%purity in dry powder form. FIG. 5B illustrates the apparatus for thepre-compacting sequence. A first die or mold 306 of desired form isfabricated at 202 to contain the boron powder. The dry boron powder isthen pre-compacted at 204 in the first die by applying ram force with apunch 302 at room temperature. The compacted boron material 304 is thenreleased upward by a static ram 308.

[0070] The pre-compacted boron powder from the first die is moved to asecond graphite die at 206 and placed in a vacuum hot press system 320(FIG. 5C). The second die 324 is lined with tantalum foil 310 of about0.005 to 0.02 inches of thickness and boron nitride powder 312 is laidbetween the tantalum foil and the boron powder 206. The tantalum andboron nitride liners function as barriers to prevent the formation ofB₄C. The boron material is heated at 208 under vacuum at the rate ofabout 50 to 300° C./hr to about 1850° C. At this temperature, pressureis applied as graphically illustrated in FIG. 5D.

[0071] The system 320 provides a cylindrical vacuum compartment 321 inwhich the boron material 304 in the second graphite die 324 is pressedby a ram 322 and ram 326. The compartment provides ports for receivingargon pressure gas 330 and outlet for vacuum pump 334. The compartmentfurther features a pressure release port and a sealed door 336 withheating elements 337. The heat is applied slowly to remove anyimpurities in the boron material and also to effect completetransformation of boron from the amorphous to the crystalline state. At1850° C. the material is held under vacuum for 1 hour and ram pressureof about 4000 to 6000 psi is applied at 210 over the boron material andsustained for about 2 hours. When ram pressure is applied, an argonoverpressure of 300-1000 Torr is maintained for the duration of theprocess. At 214, the material is then allowed to cool, still under theargon atmosphere, at the rate of about 150° C./hr. At 216, the cooledand hardened block of boron is machined and polished to a desiredconfiguration.

[0072]FIG. 6 describes a preferred hot isostatic compacting method forforming dense boron material. Again, pure boron powder, preferably of99.9% purity, is prepared at 218. The powder is typically wrapped in aflexible mold material, such as silicone rubber, and compacted at 220 atroom temperature using isostatic pressure, defined as fluid pressurewhich provides uniform pressure from all direction. The pre-compactedboron is then encapsulated at 222 with a protective foil such astitanium, tantalum or molybdenum. The boron powder is then heated at 224in a high pressure chamber to about 1500° C. to 2000° C. and sustainedfor about 1 to 3 hours. At 226, an argon pressure of at least 20000 psiis applied over the boron material. At 227, the material is allowed tocool at a controlled rate of about 300° C./hr to prevent any thermalshock to the material. At 228, the boron structure is machined andpolished to a desired configuration.

[0073] Sintering is yet another method of preparing solid boronstructure and is described in FIG. 7. Here again pure boron of 99.9%purity in dry powder form is prepared at 230. The boron powder isisostatically pre-compacted at 232 using fluid pressure at roomtemperature, similar to the pre-compacting process described in FIG. 6.The pre-compacted boron is placed in a graphite container at 234 whichis lined with tantalum foil and a layer of boron nitride powder. At 236,the container is placed in a vacuum furnace and heated to about 1950°under vacuum at the rate of 150° C./hr to about 400° C./hr. At 238 to240, while the heat is maintained at this level for about 2 to 3 hours,1 to 2 psig of argon pressure is applied. At 242, the heat level israised slowly to about 2000-2100° C. and maintained for about 1 to 3hours, still under pressure. At 244, again boron is allowed to cool at acontrolled rate of about 300° C./hr to prevent any thermal shock to thematerial. At 246, the resulting boron structure is machined and polishedto a desired configuration.

[0074]FIG. 8 describes the method of melting to form solid boron. Again,at 254 a suitable boron powder is provided. A xylene fluid is added tothe powder and the mixture is ball milled to achieve a slip of desiredconsistency. A plaster mold is fabricated at 256 to hold the boron slipin a desired shape. Prior to placing the boron powder, the mold istreated with alginate for about 60 seconds and drained at 257. At 258,the mold is air-dried for about 2 hours before being treated with xylenefor about 15 minutes at 259. The boron slip is then placed in the moldfor casting at 260. The cast boron material is removed from the mold andat 261, the boron material is heated in a boron nitride crucible undervacuum at the rate of about 300° C./hr to about 1950° C. At 262, argonpressure of 1 to 2 psig is applied over the material. At 263, thematerial is further heated to about 2200° C. and held at such a heatlevel for about 0.5 hour. At 264, the material is allowed to cool at therate of 150° C./hr to room temperature. The dense boron structure isthen machined and polished to a desired configuration at 265.

[0075] Another preferred method of making boron components for ionimplantation systems utilizes a process of pulling a boule or ingot ofsingle crystal material from a melt. This process is described in FIG.9A and schematically illustrated in FIGS. 9B and 9C. The processincludes melting boron 272 in a boron nitride crucible, pulling thesingle crystal material from the melt at 274 at a rate and temperaturesufficient to provide a desired diameter, cutting at 276 the singlecrystal material to a desired shape. The parts are then machined andpolished at 278 to form discrete components of an arc chamber or othercomponents of an implanter, and then assembled at 280 and 282 asnecessary.

[0076] The system for fabricating a single crystal material uses acrucible 360 in an oven 356 in which heaters 366 melt the boron powderto form a fluid 362. A rod 350 with a seed mounted on tip 352 contactsthe surface at 364 and is drawn up at selected speed and temperaturesuch that boule 368 is formed.

[0077] Single crystal boron can also be formed using known methods bydeposition from the vapor phase in order to form a layer of boron on anexisting part. This procedure can be used in forming filaments or incoating the internal surfaces of arc chamber or other processingchambers as described herein.

[0078]FIG. 10 describes the ion implantation process using the preferredboron materials of the present invention. A first boron filament issufficiently heated at 284 to produce boron particles inside anion-source chamber. A sufficient voltage is then applied at 286 to asecond boron filament to generate electrons inside the chamber to reactwith the boron particles. A magnetic field is generated at 288 by asource to contain such electrochemical reaction inside the chamber. Arepeller of opposite charge is provided at 290 inside the chamber tofurther contain the ions generated by the electro-chemical reaction. Theboron ions, thus formed, are extracted from the chamber by providing asufficiently small exit aperture on the chamber and extractingelectrodes near the aperture. The ion beam is then mass filtered at 294and direction-controlled by a second magnetic field in a sealed beampath at 296. The ion beam is then directed to a target for implantationat 298.

[0079] Machining dense boron materials is generally performed usingdiamond tooling including core drills, milling machines and grindingwheels. Typically cutting boron material is computer numericallycontrolled (CNC) for precision. Other methods include lapping,electrical discharge machining, laser machining, grinding, andultrasonic machining. Polishing dense boron materials typically utilizesa diamond grit paste.

[0080] Another preferred embodiment of a method for fabricatingcomponents for an ion implanter, including a boron ion source, is to usechemical vapor deposition (CVD) of boron on a substrate. In thistechnique, a boron containing gas is caused to precipitate elementalboron, either by reaction with another gas or by thermal decomposition,onto a suitably prepared substrate. The process is schematicallyillustrated in the process sequence of FIG. 11.

[0081] The first step is to prepare a substrate 518 of high density suchas a small grained graphite or other material, preferably with acoefficient of thermal expansion approximating that of boron. Thesubstrate can also be boron nitride, aluminum oxide, molybdenum,tungsten, tin, silicon carbide or other materials that can be machinedso that when a deposit of boron has been made on its surfaces, it is thedesired dimension.

[0082] A standard CVD reactor is configured so that the substrate can beheated 520 in the presence of the boron containing gas. Heating methodscan be resistive heating, induction heating, or infrared heating.Following proper preparation, positioning and heating of the substrate aboron containing gas such as boron trichloride or diborane is introduced522 to the reactor and boron is caused to deposit 524 onto the hotsubstrate by, for example, reaction of the boron trichloride withhydrogen, or alternatively, by thermal decomposition of the diborane.

[0083] The deposition temperature at which the boron is deposited ispreferably held below about 1500° C. so that the material remains in theamorphous state rather than the crystalline state. Applications in whichcrystalline materials are subjected to large thermal and/or mechanicalstresses can also result in fracture of these materials along weakerplanes within the material. This problem is eliminated by the use ofdeposited amorphous boron films. The temperature is also preferably heldabove the maximum operating temperature of the component such as an ionsource to provide the desired thermal stability in the resulting film.Thermal deposition from diborone can be conducted at about 900° C. Notethat for applications requiring the use of conductive boron films, itcan remain advantageous to use polycrystalline boron to improveconductivity. Both amorphous and polycrystalline films can also be dopedto enhance conductivity. Dopants such as silicon or carbon can bediffused or implanted during or after boron film deposition.

[0084] In one example, a plate of graphite having a coefficient ofthermal expansion of 8.2 m/m/° K. was fitted with two electrode clampsso that a current could be run through it for resistive heating. Theplate and electrodes were then placed in a reactor tube fitted with endflanges so that-it could be evacuated and then filled with flowing gasesat regulated flow rates. An infrared transducer was aimed at the surfaceof the plate for providing feedback to a temperature controller/powersupply circuit. When the graphite reached a temperature of 1240° C., theboron trichloride and hydrogen gases were introduced in the ratio of 2moles boron trichloride to 3 moles of hydrogen. After one half hours ofoperation, a uniformly thick layer measuring 0.5 mm had been depositedon the graphite plate. For preferred embodiments, it is desirable todeposit films having a thickness in the range of 0.5-3 mm.

[0085] The substrate and boron film is then removed 526 from thechamber. Depending upon the particular application or component the filmcan optionally be flurther processed by one or more methods includinggrinding or machining of one or more surfaces or regions of the film orthe substrate to achieve desired tolerances. The film can also beseparated or cleaved from the substrate.

[0086] After any optional processing one or more of the components ofthe processing chamber can be assembled 527 and installed in the ionimplanter or other device.

[0087] Illustrated in FIGS. 12A and 12B are top and cross-sectionalviews, respectively, of an ion source housing 550 fabricated inaccordance with a preferred embodiment of the invention. The chamber 550is similar in dimensions to that illustrated in connection with FIG. 2A,however, as shown along section A-A in FIG. 12B, the chamber includes aninner shell or substrate 556 on which a thin film of boron 554 has beendeposited. Deposition can be used to uniformly coat the entire surfaceincluding ports 552, or alternatively, can cover all or selectedportions of internal surfaces 558.

[0088] Shown in the top and cross-sectional views of FIGS. 13A and 13B,respectively, is the top 570 of the chamber 550. The aperture 572 canhave a beveled shape at 578, as seen in the section B-B of FIG. 13B. Athin film of boron 576 has been deposited on substrate 574 to providethe ion source aperture.

[0089] In another preferred embodiment, the deposition process can beused in the fabrication of other components described previously herein.For example, a filament or wire used in the ion source can be coatedwith a thin film of boron to provide an electrode or boron source foruse in the ion source housing. Such a film is shown in thecross-sectional view of FIG. 14 in which a metal filament 600 has a thinfilm 602 of boron thereon.

[0090] While this invention has been particularly shown and describedwith references to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims.

What is claimed is:
 1. A method of fabricating a boron structure for an ion source comprising: positioning a substrate in a processing chamber; forming a boron material on a surface of the substrate to form a composite structure; removing the boron material from the chamber; and assembling an ion source housing with the boron material.
 2. The method of claim 1 further comprising machining the composite structure.
 3. The method of claim 1 wherein the forming step comprises depositing boron in a vacuum chamber.
 4. The method of claim 1 further comprising depositing a film having a thickness in the range of 0.5 to 3.0 mm.
 5. The method of claim 1 wherein the substrate comprises graphite.
 6. The method of claim 1 further comprising forming an aperture in the substrate structure to define a beam path for ions exiting the ion source.
 7. The method of claim 1 further comprising: forming a plurality of composite boron structures; and assembling the plurality of structures to provide an ion source chamber.
 8. The method of claim 1 further comprising depositing a amorphous boron film having a density of at least 50% of maximum density.
 9. The method of claim 1 further comprising forming the boron material using a chemical vapor deposition process.
 10. The method of claim 1 further comprising providing a chemical vapor deposition reactor, providing a boron containing gas within the reactor and heating the substrate.
 11. The method of claim 1 further comprising forming an amorphous boron material on the substrate.
 12. The method of claim 10 further comprising depositing the boron material at a temperature of less than about 1500° C. to form an amorphous material.
 13. An ion source comprising: an ion source housing including an amorphous boron material having a density of at least 50% of the maximum density of boron; a cathode; an anode; and an aperture through which ions exit the ion source.
 14. The ion source of claim 13 wherein the boron material comprises a layer having a thickness in a range of 0.5 mm to 3.0 mm.
 15. The ion source of claim 13 wherein the boron material is on a substrate.
 16. The ion source of claim 13 wherein the ion source is a Bernas source.
 17. The ion source of claim 13 wherein the housing comprises a plurality of assembled boron components.
 18. A method for chemical vapor deposition of a boron structure for an ion source comprising: positioning a substrate in a chemical vapor deposition chamber; depositing a boron material in the chamber; removing the boron material from the chamber; and assembling an ion source housing with the boron material.
 19. The method of claim 18 further comprising heating a substrate in the chamber on which the boron material is deposited.
 20. The method of claim 19 wherein the heating step comprises inductively heating the substrate. 